Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EST Tue Nov 24 2020

Valid 12Z Tue Nov 24 2020 - 12Z Fri Nov 27 2020


...Pacific Northwest/Northern Rockies...
Organized, heavier precipitation is expected to return to western
Washington and Oregon by late Tuesday as a well-defined upper
level shortwave and its associated frontal band move into the
region.  This will include high elevation snow, with areas
impacted expected to include the Olympics and Cascades.  WPC PWPF
indicates that portions of the Olympics and the Washington
Cascades are likely to see accumulations of 8-inches or more by
early Wednesday. 

Showers are expected across the region through Wednesday as an
upper trough remains centered over the region.  A well-defined
shortwave dropping into the base of the trough is forecast to
produce another round of organized heavier amounts across Oregon
on Wednesday, with portions of the Oregon Cascades likely to see
an additional 6-inches during the Day 2 period.  Meanwhile, the
leading edge of the upper trough is forecast to shift east into
the northern Rockies by early Wednesday.  While widespread heavy
amounts are not expected, portions the northeastern Oregon and
southeastern Washington mountains, as well as the parts of the
northern Rockies, could see impactful amounts.

...Upper Midwest/Great Lakes...
A weak coupled upper jet, along with a low level warm front, will
support an area of rain/snow moving northeast from Iowa/Illinois
into the Upper Great Lakes region on Tuesday.  An inch or two of
wet snow is likely from southeastern Minnesota, southern Wisconsin
and Lower Michigan before precipitation ends or changes over to
rain across many locations.

...Central Rockies/Plains...
Ongoing precipitation associated with a deep upper trough moving
through the Four Corners region is forecast to continue through
the morning hours, with additional snow accumulations likely over
the Colorado Rockies and rain changing to snow farther east across
the I-25 corridor into the High Plains.  Additional accumulations
of 4-inches or more are likely over the central Colorado ranges,
with generally lighter amounts farther east.

As the upper trough moves east, models continue to show an upper
low closing off over Kansas Tuesday night.  While thermal profiles
suggest mostly rain, dynamical cooling with a developing
deformation band may support a changeover to snow, with an inch or
two of wet accumulating snow possible across portions of Kansas
into southeastern Nebraska Tuesday night.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira